1. Field of the Invention
This invention relates to an image intensifier apparatus.
2. Related Background Art
An image intensifier apparatus is an apparatus for intensifying an extremely weak optical image few ten thousands times to enable that the optical image can be visible, and which is used for a two-dimensional measurement of extremely weak light, such as a noctovision. It is assumed that this apparatus is used under the extremely weak light. Under stronger light, halo phenomenon becomes a subject of a discussion.
FIG. 1 shows this phenomenon. The halo phenomenon is a phenomenon that a limited circular area 220 of light around a spotlight appears on a fluorescent screen as a result of the bright spotlight 210 entering the photocathode of the image intensifier. The explanation of this phenomenon is described in the paper "MIL-I-49052D 3.6.9, 4.6.9, 6.3.8". FIG. 2 shows a luminous distribution in this case. In FIG. 2, the halo of 1.00 mm.O slashed. appears around the spotlight of approximately 0.15 mm.O slashed.. In a case that the halo is weak, all are relatively weak, so that no problems are arose; however, in a case that the halo is strong, some places, where no light incides, conspicuously brighten, so that picture quality is substantially lowered. This is a unique characteristic of the image intensifier, which has been requested to be improved.
As a countermeasure for the halo phenomenon, an electric method by the applicant of this invention is disclosed in Japanese Patent Publication No. 63-29781 (29781/1988) and other electric methods have been introduced. The method described in this publication is to control voltages impressed to a microchannel plate or others by detecting current of electrons toward the fluorescent screen so that this current does not exceed a certain value, and to suppress generation of surplus electrons by this control. Therefore, the halo phenomenon effect can be suppressed.
The halo phenomenon is explicated by the applicant of this invention, and the explanation is disclosed in "Japanese Patent Publication No. 33840/1990". FIG. 3 shows a mechanism of the halo, and the mechanism will be explained below. Photoelectron of the spotlight photoelectrically converted in a photocathode is accelerated and multiplied in a microchannel plate (MCP) 130. The multiplied electrons are accelerated in the acceleration electric field and strike a fluorescent screen 116, and then emit fluorescence. At this time, the electrons scatter on an aluminum metal backing evaporated on the fluorescent screen 116. Partial electrons go to the MCP 130, but they are pushed back in the acceleration electric field and reenter to the fluorescent screen 116, and then emit fluorescence. The quantity of reflected electrons on the fluorescent screen 116 is two digits lower than that of the incident electrons. However, in the case of the bright spotlight, the reflected electrons relatively increase, and then cause the halo phenomenon, so that some places, where the spotlight or other lights do not incide, brighten with the electrons scattered on the fluorescent screen 116.
In the above publication "Japanese Patent Publication No. 33840/1990", it is introduced that in order to avoid the halo phenomenon, light element such as carbon is evaporated on the aluminum metal backing evaporated on the fluorescent screen 116, so that the reflected electrons can be suppressed, and this is applied to a streak tube. The inventors of the present invention recognized that the reflected electrons can be suppressed if this method is applied to the image intensifier apparatus; however, the inventors were not satisfied with its characteristics. Thus, in order to decrease the halo phenomenon effect of the image intensifier apparatus, the inventors come to this invention.